Method for controlling a valve for an exhaust system

ABSTRACT

An exhaust gas heat exchanger system of a combustion engine has a bypass duct and a heat exchanger duct with a heat exchanger. A valve is controlled by determining when heat is to be transferred from exhaust gas flowing through the exhaust system to the heat exchanger. When heat is to be transferred, the valve is switched into a first position in which all of the exhaust gas flows through the heat exchanger duct. A pressure drop across the heat exchanger system is monitored either directly or indirectly. If the pressure drop reaches a predefined limit, the valve is switched into an intermediate position, resulting in a smaller portion of the exhaust gas flowing through the heat exchanger duct than when the valve is in the first position and a remaining portion flowing through the bypass duct, thereby reducing the pressure drop across the heat exchanger system. If the pressure drop again reaches a predefined limit, the valve is switched into a second position, resulting in an even smaller portion of the exhaust gas flowing through the heat exchanger duct, thereby further reducing the pressure drop across the heat exchanger system.

This application is a divisional of U.S. patent application Ser. No.10/930,720, filed Aug. 31, 2004, now U.S. Pat. No. 7,353,865 whichclaims priority to U.S. Provisional Patent Application No. 60/500,587filed on Sep. 5, 2003.

BACKGROUND OF THE INVENTION

The invention relates to a method for controlling a valve for an exhaustsystem of a vehicle, and in particular a valve used for controlling theflow of exhaust gas through a heat exchanger.

A heat exchanger is part of auxiliary heating systems, which areincreasingly used in vehicles having modern combustion engines with lowfuel consumption. These combustion high efficiency engines produce, onlya small amount of lost heat that is available for the heating system forthe vehicle. This results in a reduced heating performance of theheating system, which may be uncomfortable to a vehicle's occupants.Accordingly, auxiliary heating systems have been developed that use aheat exchanger located in the exhaust system of the vehicle. The heatexchanger allows a gain of a certain portion of the heat of the exhaustgas, which is then available for heating the interior of the vehicle.

Auxiliary heating systems typically have an exhaust gas duct, in whichthe heat exchanger is located, and a bypass duct. By controlling aportion of the overall exhaust gas flowing through the heat exchangerduct, a desired heating characteristic of the auxiliary heating systemcan be obtained. To this end, a valve is provided, which is controlleddepending on external parameters.

In general, a heat exchanger offers additional resistance to the exhaustgas flowing through the exhaust system, leading to a pressure drop. Thispressure drop adds to the back pressure of the exhaust system, which ingeneral is to be kept low to achieve good engine performance. Under someconditions, and in particular in cold weather conditions, the aim ofproviding better heating performance by directing the exhaust gasthrough the heat exchanger may result in a conflict with the aim ofproviding good engine performance by keeping the back pressure of theexhaust system low.

Therefore, there is a need for a method for controlling a valve in anexhaust system such that good heating performance is achieved, while atthe same time ensuring that the pressure drop across the heat exchanger,and, accordingly, the back pressure of the exhaust system does notexceed given limits.

SUMMARY OF THE INVENTION

To this end, the invention provides a method for controlling a valve inan exhaust gas heat exchanger system of a combustion engine. The exhaustgas heat exchanger system has a bypass duct and a heat exchanger ductwith a heat exchanger. The method comprises the steps of determiningthat heat is to be transferred from exhaust gas flowing through theexhaust system to the heat exchanger. Thereupon, the valve is switchedinto a first position in which all of the exhaust gas flows through theheat exchanger duct. A pressure drop across the exhaust gas heatexchanger system is monitored either directly or indirectly. If thepressure drop reaches a predefined limit, the valve is switched into anintermediate position, resulting in a first portion of the exhaust gasflowing through the heat exchanger duct and a remaining portion flowingthrough the bypass duct. Moving the valve to the intermediate positionreduces the pressure drop across the exhaust gas heat exchanger system.If the pressure drop again reaches a predefined limit, the valve isswitched into a second position, resulting in a second portion of theexhaust gas flowing through the heat exchanger duct. The second portionis smaller than the first portion, thereby further reducing the pressuredrop across the heat exchanger system.

The invention is based on the recognition that, starting from acondition in which the entire exhaust gas flows through the heatexchanger, a significant reduction of the pressure drop is achieved byallowing just a small portion of the exhaust gas to flow through thebypass duct, without resulting in a notable reduction in heatingperformance of the auxiliary heating system. In other words, thepressure drop across the heat exchanger is more sensitive to a mass flowreduction than the heating performance of the auxiliary heating system.Based on this recognition, the invention is based on the idea ofdecreasing in steps the portion of the exhaust gas flowing through theheat exchanger.

Assuming that system requirements focus on the heating performance, theposition of the valve of the exhaust gas heat exchanger system could becontrolled continuously or by using a plurality of intermediatepositions such that the pressure drop is kept below the predefinedlimit, thereby obtaining a maximum heating performance without adverselyaffecting engine performance. Assuming that system requirements focus oncosts, the position of the valve could be changed between a few definedpositions. In a preferred embodiment, only three positions of the valveare used. In this case, the second position corresponds to a position inwhich the heat exchanger is switched off and the entire exhaust gasflows through the bypass duct.

BRIEF DESCRIPTION OF DRAWINGS

The invention will hereinafter be described by means of a preferredembodiment which is shown in the enclosed drawings. In the drawings,

FIG. 1 schematically shows a first example of an exhaust gas heatexchanger system;

FIG. 2 schematically shows a second example of an exhaust gas heatexchanger system;

FIG. 3 shows a top view of the exhaust gas heat exchanger system of FIG.2;

and

FIG. 4 shows a graph indicating an example pressure drop vs. gas massflow achieved with the method according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Two embodiments of heat exchanger units employed in auxiliary heatingsystems are now described with reference to FIGS. 1 and 2. In bothsystems, the exhaust gas of a combustion engine (not depicted) enters inthe direction of arrow P from the right side with respect to thedrawings. The system has a heat exchanger duct 5 with a heat exchanger 7for the exhaust gas, and a bypass duct 9. A valve 10 is used to controlthe flow of the exhaust gas through heat exchanger duct 5 and bypassduct 9.

In the embodiment of FIG. 1, the proportion of the gas flow through theducts is controlled by varying the flow resistance of bypass duct 9.When valve 10 is in a completely opened position, the flow resistance ofbypass duct 9 is significantly less than the flow resistance of heatexchanger 7 in heat exchanger duct 5, resulting in almost no gas flowthrough the heat exchanger 7. When valve 10 is in a completely closedposition, the gas flow through bypass duct 9 is blocked, and the gasflows completely through heat exchanger 7, apart from any small leakageflow past valve 10. Moving the position of the valve 10, to intermediatepositions allows for any desired portion of the gas flow to be achievedthrough the ducts 5 and 9.

In the embodiment of FIG. 2, the gas flow is controlled directly, by avalve 110 such that the inlet opening of heat exchanger duct 5 or bypassduct 9 is opened or closed by a valve flap 11. Here again, intermediatepositions of valve 11 allow for any desired portion of the gas flow tobe achieved through the ducts 5 and 9.

FIG. 3 shows in detail the heat exchanger unit of FIG. 2. Valve 110 isactuated by an actuator 12, which controls the valve 110 between threepositions, namely a first position in which valve flap 11 closes bypassduct 9, an intermediate position approximately corresponding to theposition shown in FIG. 2 and in which valve flap 11 allows some of theexhaust gas to flow through bypass duct 9, and a second position inwhich valve flap 11 closes heat exchanger duct 5.

Actuator 12 is controlled by a ECU 14. ECU 14 is here depicted as aseparate component but could be incorporated into the control of a fuelinjection system of the combustion engine. ECU 14 is provided with asignal 16, which represents the pressure drop across the heat exchangersystem. This signal could be obtained by directly measuring the pressuredrop. Preferably, this signal is an indirect indication of the pressuredrop as the actual pressure drop depends in a known manner from otherparameters which are already known with advanced injection systems. Oneexample is to feed the ECU 14 the intake air mass flow and the amount ofinjected fuel. Based on these values, the exhaust gas mass flow can becalculated, which in turn allows for either the calculation of apressure drop across the heat exchanger system or for the gathering ofthe pressure drop based on stored values. Another example is to feed theECU 14 the engine speed, which also allows for the calculation of theexhaust gas mass flow. Other ways of indirectly determining the pressuredrop across the heat exchanger 7 could be used as well. The valve 10 inFIG. 1 can be controlled in a similar manner.

FIG. 4 shows a graph which explains the way the valve 110 is controlledbetween the first, the intermediate and the second position. In thisgraph, the x-axis generally corresponds speaking to the exhaust gasflow. The y-axis represents the pressure drop across the heat exchangersystem and therefore generally corresponds to the back pressure of theexhaust system. Broken line L represents a limit of the pressure dropand is predefined in view of acceptable back pressure of the exhaustsystem.

Upon engine start, first, there is a determination whether or not theauxiliary heating system is required. Assuming that engine start is incold weather conditions and additional heating performance is desirable,the valve 110 is switched to the first position in which the entireexhaust gas flows through heat exchanger duct 5 and thereby through heatexchanger 7. As can be seen in FIG. 4, the pressure drop and accordinglythe back pressure increases with increasing flow through the exhaustsystem (section 1 of the curve). At a point indicated with 100, the ECU14 determines that limit L of the pressure drop has been reached.Accordingly, the valve 110 is moved from the first position into anintermediate position in which a first portion of the exhaust gascontinues to flow through the heat exchanger 7 while a remaining portionflows through bypass duct 9. This results in an increase of theavailable cross section offered to the exhaust gas and a correspondingdecrease of the pressure drop (section 2 of the curve). The amount ofexhaust gas traveling through the heat exchanger duct 5 is between 50%and 90% in disclosed embodiments.

It could be shown that just a small flow of exhaust gas through thebypass duct 9 is sufficient for significantly reducing the pressure dropacross the heat exchanger system. For example, reducing the mass flowthrough the heat exchanger 7 by thirty percent (30%) may result in adecrease of the pressure drop by almost fifty percent (50%).

If the valve 110 assumes its intermediate position and the flow throughthe exhaust gas system is further increased, the pressure drop againrises (section 3 of the curve) and finally again reaches limit L (point200). Then, the valve 110 is switched into a second position in whichthe valve 110 closes heat exchanger duct 5 and entirely opens bypassduct 9, resulting in the entire exhaust gas flowing through the bypassduct 9 (possibly apart from any small leakage flow). Accordingly, thepressure drop across the heat exchanger 7 significantly decreases(section 4 of the curve).

If thereafter the flow through the exhaust gas system is furtherincreased, the back pressure again increases. However, no criticalvalues are reached as the heat exchanger 7 no longer increases the flowresistance through the exhaust gas system.

If the gas mass flow through the heat exchanger system decreases whileheat is still to be transferred from the exhaust gas to the heatexchanger 7, the valve 110 is switched back to its intermediate positionor even to its first position.

The aforementioned description is exemplary rather that limiting. Manymodifications and variations of the present invention are possible inlight of the above teachings. The preferred embodiments of thisinvention have been disclosed. However, one of ordinary skill in the artwould recognize that certain modifications would come within the scopeof this invention. Hence, within the scope of the appended claims, theinvention may be practiced otherwise than as specifically described. Forthis reason the following claims should be studied to determine the truescope and content of this invention.

1. An exhaust gas heat exchanger system, comprising: an exhaust gas flowduct having a heat exchanger duct and a bypass duct; said heat exchangerduct having a heat exchanger for providing heat to a vehicle heatingsystem; a valve separating an exhaust gas flow between said heatexchanger duct and said bypass duct, said valve having a first position,an intermediate position and a second position wherein said firstposition allows a greater portion of the exhaust gas flow through saidheat exchanger duct than said intermediate position; and a control unitcontrolling said valve, wherein said control unit is configured tocontrol said valve based upon a pressure drop relating to said heatexchanger.
 2. The exhaust gas heat exchanger system of claim 1, whereinsaid intermediate position allows a greater portion of the exhaust gasflow through said heat exchanger than said second position.
 3. Theexhaust gas heat exchanger system of claim 1, including an actuatorcontrolling said valve, said actuator being responsive to commands fromsaid control unit.
 4. The exhaust gas heat exchanger system of claim 1wherein said pressure drop is calculated from data relating to acombustion engine.
 5. The exhaust gas heat exchanger system of claim 4wherein said pressure drop is calculated from data relating to a fuelinjection system of the combustion engine.
 6. The exhaust gas heatexchanger system of claim 5 wherein said data from the fuel injectionsystem comprises intake air mass flow data.
 7. The exhaust gas heatexchanger system of claim 5 wherein said data from the fuel injectionsystem comprises an amount of injected fuel.
 8. The exhaust gas heatexchanger system of claim 1 wherein said control unit is configured toswitch said valve from said first position to said intermediate positionif said pressure drop meets a first predefined limit and to switch saidvalve to said second position if said pressure drop meets a secondpredefined limit.
 9. The exhaust gas heat exchanger system of claim 1wherein said control unit is configured to switch said valve from saidfirst position to said intermediate position if said pressure drop meetsa first predefined limit when said valve is in said first position, andwherein said control unit is configured to switch said valve from saidintermediate position to said second position if said pressure dropmeets a second predefined limit when said valve is in said intermediateposition.
 10. The exhaust gas heat exchanger system of claim 1 whereinsaid first position corresponds to a bypass duct closed position wheresaid valve closes said bypass duct to prevent a flow of gas through saidbypass duct.
 11. The exhaust gas heat exchanger system of claim 1,wherein said second position corresponds a heat exchanger duct closedposition where said valve closes said heat exchanger duct to prevent aflow of gas through said heat exchanger duct.